Bipolar junction transistors are often used in switching and amplifying applications. The basic elements of a bipolar junction transistor (BJT) include a collector, a base and an emitter. In such a structure, a first pn junction is formed at the boundary between the collector and the base and a second pn junction is formed at the boundary between the emitter and the base. The emitter and the collector are of a first conductivity type and the base is of a second conductivity type opposite to the first conductivity type.
In a high power BJT, the critical characteristics representative of the performance of the BJT are the common emitter current gain, the specific on-resistance and the blocking voltage.
Most of silicon BJTs can be manufactured using either dopant diffusion or dopant implantation, or the combination of both techniques in order to form the collector, the base and the emitter. In materials such as silicon carbide (SiC), however, dopant diffusion rates are very low and ion implantation create damage which cannot entirely be removed even at the highest available annealing temperatures. There are therefore some limitations on the processes available for manufacturing BJTs in such materials. Consequently, SiC BJTs are usually based on epitaxial designs, at least for the most critical regions of the BJT, such as the emitter and the intrinsic base. Such epitaxial designs result in a vertical topology with the collector, the base and the emitter disposed on top of each other as a stack. In such epitaxial designs, the emitter and the base are defined by etching, thereby resulting in a mesa on top of the structure. The intrinsic base corresponds to the part of the base directly located under the emitter, i.e. the part of the base interfacing the emitter.
High power BJTs in SiC are expected to provide a high blocking voltage, a low on-state voltage drop and a high emitter current gain. These parameters are, however, difficult to achieve simultaneously. Further, conventional designs and manufacturing methods are still limited with respect to reproducibility, which is a drawback for scalable commercial manufacture of such devices.
Thus, there is a need for providing new designs of high power semiconductor devices, in particular SiC BJTs, and new methods of manufacturing such high power semiconductor devices that would alleviate at least some of the above-mentioned drawbacks.